Solid vaporization



Sept. 20, 1966 R. FRANK 3,274,372

SOLID VAPORIZATION Filed July 27, 1964 //VI/E/V7'OQ DUDOLPH FPANK ewwl wqwyv ATTORNEY United States Patent 3,274,372 SOLID VAPURHZATION Rudolph Frank, La Mirada, Calif, assignor to Electro- Uptical Systems, Inc, Pasadena, Calif. Filed July 27, 1964, Ser. No. 385,509 2 Claims. (Cl. 219-271) In general, the present invention relates to a method and means for depositing a thin film of vaporized solids uniformly over a large area. More particularly, the present invention relates to a method and means of producing thin coatings of dielectric materials by thermal evaporation in a vacuum.

At present there are a wide variety of methods available whereby thin solid films may be deposited on a metallic or non-metallic surface including methods such as electrodeposition and chemical precipitation. However, the commonly used methods impose certain limitations upon the material upon which the coating is deposited, -i.e., the substrate material. For example, in the method involving the thermal decomposition of a metal carbonyl gas, the substrate must be capable of withstanding high temperature or, in the case of electro-deposition, the substrate must withstand water immersion. No such limitation is imposed on the substrate when a vacuum evaporation technique is used. Furthermore, use 'of vacuum evaporation permits the precise control of film thickness. A more complete discussion of the advantages of vacuum deposition may be found in the book by Holland entitled: Vacuum Deposition of Thin Films, (1960) pp. l-3. However, present vacuum deposition apparatus and methods have been subject in practice to certain disadvantages which have limited their application particularly with respect to the coating of large areas such as, for example, coating of a parabolic mirror to five feet in diameter. Thus, some present methods and apparatus of vacuum deposition have the tendency to scatter particles of the solid being evaporated along with the vaporized solid. More important, the present methods and apparatus are capable of depositing uniform coating thicknesses over only small areas and thus normally require multiple sources in order to deposit a uniform thickness over a large area. Such limitations and disadvantages are found if the examples of the current vaporizer units illustrated by Holland, supra, on page 119 are used, particularly when granular dielectric materials such as silicon monoxide, zinc sulfide, magnesium fluoride, cryolite, and others are vaporized by sublimitation. Such illustrated prior Vaporizers, besides having the aforementioned disadvantages, also require elaborate shielding to prevent radiation heating of the chamber and work pieces as well as large power consumption. Furthermore, current attempts to remedy such disadvantages has produced relatively expensively complicated equipment without satisfactorily solving the problem, for example, of uniform deposition over a large area.

Consequently, an object of the present invention is a simple, efficient solid vaporizer adapted to deposit a thin film of vaporized solid uniformly over a large area.

Another object of the present invention is a solid vaporizer for granular dielectric material which is adapted to vaporize the solid without the emission or scattering of solid particles.

Still another object of the present invention is a method of vaporizing solids to deposit a thin film of vaporized solid uniformly over a large area without the emission of solid particles.

Still another object of the present invention is an apparatus for vaporizing solids which has low power con sumption and does not require extensive shielding of the vaporization chamber and work pieces.

Still another object of the present invention is a simple,

eflicient solid vaporizer adapted to vaporize solids over a longer period of time without recharging.

Still another object of the present invention is a solid vaporizer adapted to achieve very rapid deposition of the vaporized solid on a substrate.

Other objects and advantages of the present invention will bev readily apparent from the following description and drawings which illustrate a preferred exemplary embodiment of the present invention.

In general, the present invention involves a simple, efficient solid vaporizer conmprising a shallow refractory container having a large area open top. Mounted in such container is a series of spaced helical coils which extend substantially parallel to the bottom wall of the container and are adapted to be completely enclosed by the solid charge in the container. The method of the present invention involve forming a solid charge having a large open top surface area and then heating such charge internally adjacent to, but spaced from, its top surface over substantially the same area as the top surface area.

In order to facilitate understanding of the present invention, reference will now be made to the appended drawings of a preferred specific embodiment of the present invention. Such drawings should not be construed as limiting the invention which is properly set forth in the appended claims.

In the drawings: 7

FIG. 1 is a prospective view of a specific embodiment of the solid vaporizer of the present invention.

FIG. 2 is a cross sectional view of FIG. '1 taken along the lines 2--2 of FIG. 1.

As illustrated in FIGS. 1 and 2, the vaporizer 10 comprises a shallow refractory container 11 having a large area open top 12. The container 11 includes a bottom wall 13, and a set of side walls 14, 15, '16, and 17 forming a square. In the opposed pair of side walls 14 and 16, is a series of equally spaced aligned holes '18. Mounted in the holes are insulator plugs 19 having axial bores 20 adapted to receive a wire and formed out of material such as boron nitride. Mounted in the container 11 is a series of spaced helical coils 21 which extend substantially parallel to the bottom wall '13 of the container. I he coils 2 1 are adapted to be completely enclosed by a solid charge 30 in the container which fills the container adjacent to the top edge of the side walls 14, 15, 16, and 17. Thus, the coils 21 are mounted substantially half way between the top and bottom of the container 11. Furthermore, the coils 21 are adapted to heat the solid charge in the container internally over substantially the same area as the top surface area. Thus, the coils are mounted substantially parallel to each other and are substantially equally spaced from each other and from the parallel side walls. The wire ends 22 of the coils 21 extend through the bores 20 of the insulator plugs 19 so that the coils 2.1 are supported thereby.

The operation of the solid vaporizer illustrated in FIGS. 1 and 2 illustrates its simplicity and efficiency when in use. Thus, the method of the pres-ent invention involves forming a solid charge 30 in the container 11 having a large open surface area such as illustrated. Electric current is then passed through the coils to heat the charge internally adjacent to, but spaced from, its top surface over substantially the same area as the top surface. Thus, as illustrated, the charge is formed having a shallow depth with the heating done substantially half way between the top and bottom of the charge. Under such conditions solids such as dielectric materials are sublimated uniformly over a wide solid angle with particle emission.

A solid vaporizer was constructed as illustrated in FIGS. 1 and 2 out of a sheet of tantalum 0.015 inch thick so that the container was two inches square at the top and had a depth of one inch. A set of three tungsten wires 0.040 inch in diameter forming helical coils approximately /8 inch in diameter was mounted in the container and extended between the end walls perpendicular to the axis of the helical coil. The axis of each of the three helical coils was equally spaced /2 inch from the parallel side walls and from each other. Similarly, the helical coils were spaced approximately /2 inch from the bottom wall or approximately half way between the top and bottom of the container. Such container was filled uniformly to within about A; inch of its top edge with granular silicon monoxide and then used to vaporize and deposit a coating of silicon monoxide on the parabolic surface of a mirror five feet in diameter. The resulting coating was found to be substantially uniform over the entire surface of the mirror. In addition, it was found that such solid vaporizer is capable of depositing silicon monoxide at the rate of approximately angstroms/ second on a substrate approximately 750 mm. from the top surface.

Many other specific embodiments of the present invention will be obvious to one skilled in the art in view of this disclosure. For example, although the shape of the container has been illustrated in the form of a square, a rectangular or circular container may be utilized. Like- Wise, although the use of insulator plugs is preferred to minimize the radiant heat loss from the interior of the container, the wires may be simply suspended in the holes in the side walls of the container so that they do not contact the container.

There are many features in the present invention which clearly show the significant advance the present invention represents over the prior art. Consequently, only a few of the more outstanding features will be pointed out to illustrate the unusual and unexpected results obtained by the present invention. One feature of the present invention is the arrangement of the heating coils with respect to the solid charge to be vaporized. As disclosed above, the heating coils and charge are arranged so that there is a substantially equal thickness of charge coating around each of the heating coils. Consequently, during the evaporation of the solid, a lining is formed over the inner surfaces of the container which acts to reduce the heating of the container. Also, a cavity forms immediately around the heating coil wherein most of the heater power is utilized in the production of vapor. In this way, the power input to the vaporizer is utilized in an eflicient manner to produce vapor rather than heating up the surrounding equipment. Another feature of the present invention is the emission of the solid vapor substantially uniformly over a wide solid angle from the surface of the vaporizer. Consequently, parabolic surfaces up to five feet in diameter have been coated with silicon monoxide by a single vaporizer of the present invention with substantial uniformity. Still another feature of the present invention is that vaporization occurs internally within the charge and the resulting vapor is made to diffuse through a covering crust of granules. Thus, the ejection of solid particles is eliminated and the charge is heated uniformly. Still another feature of the present invention is that substantially no outgassing of the chambet and associated equipment occurs during the use of the vaporizer of the present invention. Still another feature of the present invention is the ability of the vaporizer to deposit a uniform coating very rapidly.

It will be understood that the foregoing description and examples are only illustrative of the present invention and it is not intended that the invention be limited thereto. All substitutions, alterations, and modifications of the present invention which come within the scope of the following claims or to which the present invention is readily susceptible without departing from the spirit and scope of this disclosure are considered part of the present invention.

I claim:

1. A simple, efficient vaporizer adapted to deposit a thin film of vaporized solid uniformly over a large area without the emission of solid particles comprising:

(a) a shallow refractory container having a large area open top, said container having a top area of at least about four square inches and a depth of less than about one inch; and

(b) a series of at least three spaced helical coils mounted in said container substantially halfway between the top and bottom of said container and extending substantially parallel to the bottom wall of said container and substantially parallel to each other, said coils being adapted to be completely enclosed by the solid charge in said container and to heat said solid charge internally adjacent to but spaced from its top surface over substantially the same area as the top surface.

2. A method of vaporizing solids to deposit a thin film of vaporized solid uniformly over a large area without the emission of solid in a vacuum particles comprising:

(a) forming a solid charge having a large open top surface area of at least about four square inches and a shallow depth of less than about one inch; and

(b) heating said solid charge internally adjacent to but spaced from its top surface over substantially the same area as the top surface and substantially halfway between the top and bottom of said charge so that said charge is heated substantially uniformly.

References Cited by the Examiner UNITED STATES PATENTS 1,110,532 9/1914 Byce 2l9395 1,456,141 5/1923 Morrison 2l9-2'75 2,674,973 4/1954 Thorington 11849 RICHARD M. WOOD, Primary Examiner.

ANTHONY BARTIS, Examiner.

C. L. ALBRITTON, Assistant Examiner. 

1. A SIMPLE, EFFICIENT VAPORIZER ADAPTED TO DEPOSIT A THIN FILM OF VAPORIZED SOLID OVER A LARGE AREA WITHOUT THE EMISSION OF SOLID PARTICLES COMPRISING: (A) A SHALLOW REFRACTORY CONTAINER HAVING A LARGE AREA OPEN TOP, SAID CONTAINER HAVING A TOP AREA OF AT LEAST ABOUT FOUR SQUARE INCHES AND A DEPTH OF LESS THAN ABOUT ONE INCH; AND (B) A SERIES OF AT LEAST THREE SPACED HELICAL COILS MOUNTED IN SAID CONTAINER SUBSTANTIALLY HALFWAY BETWEEN THE TOP AND BOTTOM OF SAID CONTAINER AND EXTENDING SUBSTANTIALLY PARALLEL TO THE BOTTOM WALL OF SAID CONTAINER AND SUBSTANTIALLY PARALLEL TO EACH OTHER, SAID COILS BEING ADAPTED TO BE COMPLETELY ENCLOSED BY THE SOLID CHARGE IN SAID CONTAINER AND TO HEAT SAID SOLID CHARGE INTERNALLY ADJACENT TO BUT SPACED FROM ITS TOP SURFACE OVER SUBSTANTIALLY THE SAME AREA AS THE TOP SURFACE. 